Boron coating, or "boronization" of tokamaks, with either chemical vapor deposition of boron from boron containing compounds, or "boronization" from evaporation of solid targets of boron/carbon mixtures, has produced beneficial results. Boron has properties that are advantageous to improved reactor performance. As a material that comes in contact with the plasma edge, it has been found to reduce the influx of some materials, the retention of hydrogen (tritium), the oxygen and oxygenated species, and even lower the loop voltage. The presence of boron also helps maintain the low atomic number preferred for a tokamak coating. Additionally, the use of boron coatings may have widespread applicability in vacuum systems for electrical and/or corrosion resistance in contained environments with controlled atmospheres under non-vacuum conditions.
Diborane, a highly toxic pyrophoric gas, is presently used for boronization of certain substrates or chambers, such as the TFTR at Princeton Plasma Physics Laboratory. However, it leaves residual hydrogen, as do other hydrogenous boron compounds unless they are completely reacted. Diborane is also used in chemical vapor deposition systems in the semiconductor industry. For use in tokamaks, safety dictates that the handling of this gas is not permitted as often as desired, and thus diborane is not suitable for boronization processes as frequently as needed.
Pure boron has a resistivity of .apprxeq.10.sup.6 ohm-cm at room temperature and is therefore not a good candidate for conventional D.C. sputtering. Because boron is a non-conductor at room temperatures, it has not been sputtered except by costly radio-frequency plasma sources. RF sources that operate at 13.6 MHz are commercially available. These sources are about one order of magnitude more expensive than conventional direct current sources, and have critical proximity limitations with the sputter target. Although boron is nonconductive at room temperature, it has a unique property of becoming more conductive at higher temperatures: its resistivity changes to .apprxeq.10 ohm-cm at 400.degree. C. This property of boron can be exploited to provide a conductive material that can be sputtered with direct current.
Accordingly, it is an object of the present invention to provide an alternative process for the boronization of substrates, such as the walls of tokamak chambers, which avoids the safety problems exhibited by conventional boronization methods involving diborane or other hydrogenous boron compounds.
It is another object of the present invention to provide a less costly alternative to conventional methods of boronizing certain substrates.
Another object of the present invention is to provide a sputtering process for the coating of substrates with boron that takes advantage of the unique conduction characteristics of this element.